1. Field of the Invention
The present invention relates generally to a video tape recorder and/or reproducer, and more particularly to an improved tracking control device and circuit for adjusting the tracking of a rotary head assembly during video signal reproduction.
2. Description of the Prior Art
A video tape recorder and/or reproducer, which records a video signal on a magnetic tape by a rotary magnetic head assembly and/or reproduces the video signal from the magnetic tape by the head assembly, is well known.
In such an apparatus, when the video signal is recorded, the rotation of the rotary magnetic head is phase locked to the vertical synchronizing signal contained in the video signal. Accordingly, it is necessary to adjust the phase of rotation of the rotary magnetic head assembly to attain phase lock. Similarly, when a video signal recorded on the magnetic tape is reproduced, the rotary magnetic head assembly must correctly trace or scan the recording track of the video signal on the magnetic tape. This is obtained by phase adjusting the rotation of the rotary magnetic head assembly or the driving of a capstan which drives or transports the magnetic tape.
To perform the above phase adjustment, it has been proposed to employ a pulse generator for indicating the position of the rotary magnetic head assembly. The pulse generator includes a pole piece which rotates with the rotary magnetic head assembly and a pick-up which senses the position of the pole piece when the rotary magnetic head assembly arrives at a predetermined position, for example, at a position where the head first contacts the magnetic tape. Further, there are provided a separator for separating the vertical synchronizing signal from the video signal when the latter is recorded, a phase comparator which detects the phase difference between the separated vertical synchronizing signal and the pulse signal produced by the pulse generator, a head assembly drive control for achieving a phase adjustment of the rotation of the rotary magnetic head in accordance with a phase control signal proportional to the phase difference detected by the phase comparator, and a control signal recording head for recording a control signal representing the separated vertical synchronizing signal on the magnetic tape.
When recording, the rotary magnetic head assembly is rotated in phase lock relation to the vertical synchronizing signal by the above apparatus and the vertical synchronizing signal or a signal in synchronous association therewith is recorded by the control signal recording head. When reproducing the video signal from the magnetic tape, a servo control operation is carried out such that the control signal recording head is used as a signal reproducing head to reproduce the control signal from the tape which is driven by a capstan. The reproduced control signal and a pulse signal produced by the pulse generator are both applied to the phase comparator, which, in turn, produces a phase control signal proportional to the phase difference between the reproduced control signal and the pulse signal to control the rotation of the rotary magnetic head assembly or the capstan so as to minimize the phase difference.
In practice, however, there are some instances during a signal reproduction operation when the rotary magnetic head can not precisely scan or trace the recording track of the video signal on the magnetic tape in response to the above servo control. Various causes can be considered for this defect. By way of example, the mounting position of the control signal recording and/or reproducing head of the recorder or reproducer may shift from a determined precise position, the magnetic tape having the video signal recorded thereon may expand or shrink since the signal had been recorded, and so on. Additionally, when a video signal has been recorded by a different video tape recorder, the rotary head often cannot accurately trace the prerecorded signal tracks.
In order to avoid such defect and to provide that the rotary magnetic head correctly traces the recording track of the video signal on the tape, a prior art video tape recorder has included with a tracking adjustment means for the rotary magnetic head. Such tracking adjustment means includes a time delay means such as a monostable multivibrator inserted between the pulse generator and the phase comparator, the delay time of the delay means being changed manually, as is well known. In the prior art tracking adjustment means, the pulse signal from the pulse generator is supplied to a first time delay means to be delayed by a time interval T.sub.H, the time required for the rotary magnetic head to be rotated by a half rotation, and the delayed pulse is then further supplied to a second time delay means to be further delayed by an interval (T.sub.H .+-.T.sub.H) or 0.about.2 T.sub.H, and the output signal from the second time delay means is fed to the phase comparator. Thus, the tracking adjustment is achieved by manually varying the delay time of the second time delay means. In other words, the prior art tracking adjustment is carried out in such a manner that the pulse signal from the pulse generator is delayed by a time longer or shorter than the time which is required for the rotary magnetic head to rotate one turn and the delayed pulse signal is thereafter applied to the phase comparator.
However, this prior art technique of tracking adjustment is accompanied by noticeable defects. The servo control apparatus, which provides for the rotary magnetic head to correctly trace the recording track of the video signal on the tape while a tracking adjustment is being carried out, operates on the assumption that a pulse signal is received from the pulse generator at a time less than one rotation time of the rotary magnetic head. Accordingly, even if any change of the rotation speed of the rotary magnetic head or the travelling speed of the tape occurs during one rotation time period of the rotary magnetic head, the servo control apparatus of the above type can not compensate for such a change in speed. As a result, noise may increase in the reproduced video signal due to incorrect trace. Also, in the prior art a monostable multivibrator is used as the time delay means because its delay time can be easily varied. However, in practice the recovery time of he monostable multivibrator may drift because of variations of power source voltage or ambient temperature, so that the tracking adjustment must be re-adjusted often. In accordance with the prior art technique it is very difficult to provide a desired delay time of the time delay means that is relatively free from the influence of drift in the recovery time of the monostable multivibrator. Further, the use of two monostable multivibrators connected in a cascade manner for obtaining the desired delay time results in a complex circuit requiring a larger number of adjustable elements.